Preparation of conductive polymers from stabilized precursor solutions

ABSTRACT

Conductive polymers are prepared from a stabilized solution containing a monomer, an Fe(III) oxidizing agent, and a mixed solvent. The solvents are selected to stabilize the Fe(III) oxidizing agent and monomer in solution while allowing highly conducting polymers to be produced upon evaporating the lower-boiling solvent. The higher-boiling solvent does not appreciably complex with Fe(III), while the lower-boiling solvent forms a weak complex with Fe(III). The mixed-solvent system of the present invention may be used for preparing a conductive polymer counter electrode in a solid tantalum capacitor by polymerizing the monomer inside a porous tantalum pellet.

This application is a division of Ser. No. 09/321,740, filed May 28,1999, which application is a divisional of Ser. No. 09/148,484, filedSep. 4, 1998 now U.S. Pat. No. 6,001,281.

FIELD OF THE INVENTION

The present invention relates to stabilized solutions useful for thesynthesis of conductive polymers. Conductive polymers can be chemicallysynthesized from a monomer, an oxidizing agent, and a dopant. Normallysuch solutions have a very limited useful life because the monomerreacts with the oxidizing agent in the solution. The solutions may beused for the production of conductive polymer cathodes for solidtantalum capacitors.

BACKGROUND OF THE INVENTION

A solid state electrolytic capacitor is made from a porous pellet ofsintered tantalum powder, a dielectric tantalum oxide layer formed onthe surface of the sintered tantalum powder, a solid-state conductorimpregnated into the volume of the pellet, and external connections suchas silver paint, etc. The tantalum forms the positive electrode of thecapacitor and the solid-state conductor forms the negative electrode(also called the cathode or counter-electrode).

Manganese dioxide has been employed as the solid-state conductor intantalum capacitors for the last 50 years. To impregnate the pellet withmanganese dioxide, it is dipped into a solution of manganese nitrate.The pellet is then heated in air or steam to temperatures in excess of200° C. to pyrolyze the manganese nitrate to manganese dioxide. Thisprocess of dipping and pyrolysis is repeated several times to fill thepellet with manganese dioxide. By carefully choosing the sequence ofconcentrations of manganese nitrate and the pyrolysis conditions, acapacitor with a high capacitance recovery and a high volume fill ofmanganese dioxide can be produced.

A key property of manganese dioxide is its self-healing ability. Atdefective portions of the dielectric film, the manganese dioxide becomesnon-conductive. This is due to the manganese dioxide transforming to alower manganese oxide because of joule heating at the defect site. Thismechanism allows capacitors with low leakage currents to be produced. Italso allows small dielectric defects that occur during manufacture anduse to be isolated. However, if the dielectric defect is too large, thedielectric can crack. Manganese dioxide is a powerful oxidizing agent.When it comes in direct contact with tantalum through a crack in theoxide, the capacitor can ignite, leading to destruction of the capacitorand possible destruction of other components in the circuit. It isdesirable to replace the manganese dioxide with a solid-state conductorthat is non-oxidizing, therefore eliminating tantalum ignition whilemaintaining the self-healing mechanism.

The use of tantalum capacitors in high frequency circuits has becomemore important. This has led to the need for tantalum capacitors havinglow equivalent series resistance (ESR). The best manganese dioxide has aresistivity of 0.5 to 10 ohm-cm. It is desirable to replace themanganese dioxide with a solid-state conductor that has a lowerresistivity. However, many highly conductive metals and oxides do nothave a self-healing ability and thus are not suitable for solid-statetantalum capacitors.

Conductive polymers such as polypyrroles, polyanilines, andpolythiophenes have resistivities 10 to 100 times less than that ofmanganese dioxide. Since they are much less powerful oxidizing agentsthan manganese dioxide, these materials do not cause the capacitor toignite upon failure. Polypyrrole was shown to have a self-healingmechanism (Harada, NEC Technical Journal, 1996).

Chemical oxidative polymerization is an effective way to impregnateconductive polymer into the pores of the tantalum pellet. In chemicaloxidative polymerization, a monomer, oxidizing agent, and a dopant arereacted inside the porous pellet to form the conductive polymer.Monomers include pyrrole, aniline, thiophene, and various derivatives ofthese compounds. The oxidizing agent can be either an anion or a cation.Typical anion oxidizers are persulfate, chromate, and permanganate.Typical cations are Fe(III) and Ce(IV). The best dopants are anions ofstrong acids such as perchlorate, toluenesulfonate,dodecylbenzenesulfonate, etc. The reaction between monomer, oxidizingagent, and dopant can take place in a solvent such as water, an alcohol,a nitrile, or an ether.

Several methods have been used to get the monomer, oxidizing agent, anddopant into the porous pellet and carry out the conversion to conductivepolymer. In one method, the pellet is first dipped in a solution of theoxidizing agent and dopant, dried, and then dipped in a solution of themonomer. After the reaction is carried out, the pellet is washed andthen the process is repeated until the desired amount of polymer isdeposited in the pellet. In this method, it is difficult to control themorphology of the final polymer. It is also difficult to control theexact reaction stoichiometry between the monomer and the oxidizingagent. Control of this stoichiometry is critical to achieve the highestconductivity polymer (Satoh et al., Synthetic Metals, 1994). Crosscontamination of the dipping solutions is a problem. Since the pelletmust be dipped twice for each polymerization, the number of processsteps is greatly increased. The excess reactants and the reduced form ofthe oxidizing agent need to be washed out of the part. This adds evenmore process steps and complexity to the process.

In a related method, the sequence is reversed so that the pellet isdipped in the monomer solution first and the solvent is evaporated away.The pellet is then dipped in the oxidizing agent/dopant solution and thereaction is carried out. This method suffers from all the disadvantagesof the previous method. In addition, some monomer may be lost in thesolvent evaporation step.

In the preferred method, all components are mixed together and thepellet is dipped in the combined solution. This method reduces thenumber of dips and allows more precise control over the reactionstoichiometry. However, the monomer and oxidizing agent can react in thedipping bath, causing premature polymerization and loss of reactants,adding some cost and complexity to the process. This is especially aproblem with pyrrole monomer and Fe(III) oxidizing agents. To partiallyovercome this, the dipping bath can be kept at cryogenic temperature(Nishiyama et al., U.S. Pat. No. 5,455,736). However, use of cryogenictemperatures adds considerable equipment and operational complexity tothe process.

The pyrrole/Fe(III) can be replaced with a monomer/oxidizing agentcombination that is less reactive. For example, 3,4ethylenedioxythiophene and an Fe(III) salt of an organic acid may bedissolved in alcohol or acetone (Jonas et al., U.S. Pat. No. 4,910,645).With this combination, dilute solutions (less than 5% monomer) arestable near room temperature for several hours. The polymer (poly (3,4ethylenedioxythiophene) or PEDT) can be formed by warming the solution.At concentrations greater than about 5% monomer, the components reactquickly. Cooling the dipping solution can be used to retard thereaction. However, there is a lower limit to which the solution can becooled because of limited solubility of the components at lowtemperatures. Addition of a nonvolatile organic base, such as imidazole,also inhibits the reaction (Mutsaers et al., EP 0615256 A2, 1994; deLeeuw et al., 1994). However, this leaves an organic residue in thepores of the tantalum capacitor that is difficult to wash out.

Because of the trend toward higher surface area (higher charge) tantalumpowders, it is desirable to use more concentrated monomer/oxidizingagent/doping solutions to more effectively cover the internal surfacearea of the tantalum pellet in a reasonable number of dips. Theinstability of solutions of monomers/oxidizing agents/dopants at higherconcentrations is an impediment to achieving that objective.

It is known in the art that the choice of solvent can greatly affect thereaction rate between the monomer and the oxidizing agent. For example,Myers (J. Electron. Mater., 1986) prepared polypyrrole from pyrrole andFe(III) chloride in several different solvents. The preparation was doneby mixing the components in solution and allowing the reaction to takeplace without evaporation of the solvent. The highest yields ofpolypyrrole were obtained in solvents which did not appreciably complexwith Fe(III). Solvents which showed a strong exotherm on addition ofFe(III) gave very low yields.

More recently, Lebedev et al. (Chem. Mater., 1998) studied the reactionbetween an Ag(I) salt and neutral forms of polythiophene derivatives.The Ag(I) oxidizes the neutral polymer to an oxidized conducting form ofthe polymer. The counter ion of the salt becomes the polymer dopant.This reaction takes place quickly in solvents such as xylene, toluene,heptane, and chloroform. In order to stabilize such solutions, Lebedevet al. use a mixed solvent system of toluene, heptane and pyridine. Thepyridine complexes with Ag(I) and prevents the reaction in solution.When the pyridine is allowed to evaporate from the mixed solvent system,the Ag(I) oxidizes the polymer to a conducting form. The system ofLebedev et al. is not suitable for use in preparation of capacitors forseveral reasons: the Ag(I) reactant is expensive, the Ag(O) reactionproduct would be difficult to remove, and the neutral polythiophenes areonly soluble in low concentrations.

A mixed solvent system is also taught by Nakama (U.S. Pat. No.5,514,771, 1996). Ion-doped poly(alkyl-substituted pyrrole) with aconcentration of greater than 5% is prepared in an organic solvent andmay be used as a solid electrolyte in a capacitor. Alkyl-substitutedpyrrole is dissolved in a solvent comprising THF, aliphatic alcohols, ormixtures thereof An oxidizing agent such as a ferric (III) saltpolymerizes the alkyl-substituted pyrrole, and water is added toseparate the product by filtration. The mixed solvent system dissolvesthe reactants and does not prohibit the polymerization reaction.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to stabilize mixedsolutions of a monomer, an Fe(III) oxidizing agent, and dopant againstpolymerization by dissolving the monomer, Fe(III) oxidizing agent, anddopant in a mixture of two solvents where the higher-boiling solventdoes not appreciably complex with Fe(III) and the lower-boiling solventforms a weak complex with Fe(III).

It is another object of the present invention to develop a mixed solventsystem for preparing conductive polymers which eliminates the need forremoving a complexing agent by washing.

It is another object of the present invention to prepare conductivepolymers by evaporating the low-boiling solvent followed by reacting themonomer, Fe(III) oxidizing agent, and dopant in the higher-boilingsolvent.

It is yet another object of the present invention to prepare conductivepolymers using a mixed solvent system without the need for removing acomplexing agent by washing.

It is yet a further object of the present invention to deposit aconductive polymer in a tantalum pellet by dipping the pellet in a mixedsolvent system which is highly stable at room temperature, andevaporating the lower-boiling solvent to allow the monomer to react withthe oxidizing agent.

According to one aspect of the present invention, a stabilized precursorsolution for preparing conductive polymers comprises:

(a) a monomer;

(b) an Fe(III) oxidizing agent;

(c) a first solvent having a first boiling point, wherein the firstsolvent does not appreciably complex with Fe(III); and

(d) a second solvent having a second boiling point, wherein the secondsolvent forms a weak complex with Fe(III);

wherein the first boiling point is higher than the second boiling point.

The monomer may be pyrrole, thiophene, or derivatives thereof such as3,4 ethylenedioxythiophene. The high-boiling solvent may be a ketone orlower alcohol such as 1-butanol. The lower-boiling solvent may be adioxane or a cyclic ether such as tetrahydrofuran. A suitable oxidizingagent is Fe(III) tosylate.

According to another aspect of the present invention, a process forpreparing a conductive polymer from a stabilized precursor solutioncomprises:

(a) dissolving a monomer and an Fe(III) oxidizing agent in a mixedsolvent, the mixed solvent comprising a first solvent having a firstboiling point, wherein the first solvent does not appreciably complexwith Fe(III), and a second solvent having a second boiling point whichis lower than the first boiling point, wherein the second solvent formsa weak complex with Fe(III);

(b) evaporating the second solvent; and

(c) reacting the monomer and oxidizing agent to produce a conductivepolymer.

According to yet another aspect of the invention, a process fordepositing a conductive polymer in a porous pellet comprises:

(a) providing a porous pellet made from a material selected from thegroup consisting of tantalum, aluminum, niobium, zirconium, and hafnium;

(b) anodizing the pellet;

(c) dipping the pellet into a solution comprising a monomer, an Fe(III)oxidizing agent and a mixed solvent, the mixed solvent comprising afirst solvent having a first boiling point, wherein the first solventdoes not appreciably complex with Fe(III), and a second solvent having asecond boiling point which is lower than the first boiling point,wherein the second solvent forms a weak complex with Fe(III);

(d) evaporating the second solvent;

(e) reacting the monomer with the oxidizing agent to form a conductivepolymer; and

(f) washing the pellet.

It has surprisingly been found that certain combinations of solventswill allow highly conducting polymers to be produced from the reactionsof Fe(III) salts and monomer while, at the same time, stabilizing thereactants in solution prior to reaction. A mixture of two solventswherein the higher-boiling solvent does not appreciably complex withFE(III) and the lower-boiling solvent forms a weak complex with Fe(III)has been found to be effective for stabilizing the monomer yet enablinghighly conductive polymers to be produced by evaporating thelow-boiling, complexing solvent followed by reaction of the monomer,Fe(III) oxidizing agent, and dopant in the higher-boiling solvent.Because the lower-boiling solvent is completely removed by evaporation,there is no excess complexing agent to remove by washing.

DETAILED DESCRIPTION OF THE INVENTION

Suitable monomers for preparing conductive polymers which may be used inaccordance with the present invention include but are not limited topyrrole, thiophene, and derivatives thereof. Monomers for preparingconductive polymers are well known in the art, for example as taught byU.S. Pat. No. 4,910,645 to Jonas et al., incorporated by referenceherein. A preferred monomer is 3,4 ethylenedioxythiophene.

Fe(III) oxidizing agents for preparing conductive polymers are also wellknown in the art. Jonas et al. teach various oxidants for thepolymerization of thiophenes and pyrroles, which oxidants includeFe(III) salts of organic and inorganic acids, alkali metal persulfates,ammonium persulfates, and others. The preferred oxidant is Fe(III)tosylate.

The high-boiling solvent may be selected from solvents in which themonomer, Fe(III) salt and dopant are soluble and which do not interferewith the oxidization of the monomer by Fe(III). Examples of thehigher-boiling solvent are lower alcohols and ketones. 1-butanol is apreferred solvent because iron salts such as Fe(III) tosylate andmonomers such as pyrrole, thiophene and derivatives have goodsolubility.

The lower-boiling solvent should form a weak-to-moderate complex withFe(III) so as to retard the reaction between Fe(III) and the monomer.However, the lower-boiling solvent should not complex the Fe(III) sostrongly that it forms a stable complex upon evaporation. Examples oflower-boiling solvents that form weak complexes with Fe(III) are cyclicethers. It has also been discovered that 1,4 dioxane is effective forstabilizing the monomer. Preferred ethers are unsubstituted cyclicethers such as tetrahydrofuran and tetrahydropyran. The most preferredether is tetrahydrofuran.

The difference between boiling points of the low-boiling solvent and thehigh-boiling solvent is preferably at least about 5° C., more preferablyfrom about 5° C. to about 60° C. Most preferably the difference inboiling points is at least from about 15° C. to about 60° C.

The monomer may be provided in a concentration of from about 0.5 toabout 12 percent by weight, preferably about 6 percent by weight. Thehigher-boiling solvent may be provided in a concentration of from about10 to about 80 percent by weight, preferably about 16 percent by weight.The lower-boiling solvent may be provided in a concentration of fromabout 10 to about 80 percent by weight, preferably about 50 percent byweight.

A conductive polymer film may be produced by heating the solution to atemperature of from about 25° C. to about 160° C., preferably about 110°C. The heat should be applied for a time sufficient to evaporate thelow-boiling solvent while facilitating the polymerization reactionbetween the monomer and Fe(III) oxidizing agent.

A conductive polymer cathode for a solid tantalum capacitor may beprepared by pressing charged tantalum powder into pellets and sinteringto form a porous body. The pellets are then anodized in a phosphoricacid electrolyte at a voltage from about 14V to about 140V. Conductivepolymer is then deposited in the anodized tantalum pellets bypolymerization of the dipping solution at a temperature of from about25° C. to about 160° C., preferably about 110° C., followed by washingin water or another solvent. The anodes may be reformed, e.g., afterevery 4th dip, in a 1% aqueous solution of p-toluenesulfonic acid. Aswill be appreciated by those skilled in the art, various other materialsmay be used for forming the capacitor, such as aluminium, niobium,zirconium, and hafnium.

EXAMPLES

Reference to the following illustrative examples is made for a morecomplete understanding of the invention. These examples are illustrativeof preferred aspects of the invention and are not intended to limit thescope of the invention.

Example 1

Solutions of 3,4 ethylenedioxythiophene (monomer), Fe(III) tosylate(oxidizing agent and dopant), and solvent(s) were made according to thecompositions given in Table 1. The monomer was obtained from Bayer underthe trade name Baytron M, and the Fe(III) tosylate was obtained fromBayer as a 50 wt % solution in 1-butanol (trade name Baytron C). Herethe 1-butanol is the high-boiling solvent (BP 117° C.). The othersolvent was selected from the classes of alcohols, ketones, cyclicethers, and amines that have a boiling point lower than that of1-butanol.

Each of the solutions was placed in a closed vial. The initial color ofthe solutions ranged from yellow-brown to red-brown. The solutions wereallowed to stand at room temperature for four hours. After this timeperiod, the solutions were examined visually for color changes and alsofor the presence of precipitated PEDT powder. As the polymerizationreaction proceeded, the color of the solution became blue, and blueprecipitate of PEDT polymer formed.

                  TABLE 1                                                         ______________________________________                                        Solution Compositions (by weight %)                                           Component      A     B       C   D     E    F                                 ______________________________________                                        3,4 ethylenedioxythiophene                                                                   4     4       4   4     4    4                                 Fe(III) tosylate                                                                             16    16      16  16    16   16                                1-butanol      80    16      16  16    16   16                                2-propanol     0     64      0   0     63.4 0                                 Methanol       0     0       64  0     0    0                                 Acetone        0     0       0   64    0    0                                 Pyridine       0     0       0   0     0.6  0                                 Tetrahydrofuran                                                                              0     0       0   0     0    64                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Reactions of Solutions at Room Temperature                                    Solution                                                                              Initial Color Color at 4 Hours                                                                          Precipitate                                 ______________________________________                                        A       Yellow-Brown  Blue        Yes                                         B       Yellow-Brown  Blue        Yes                                         C       Yellow-Brown  Blue        Yes                                         D       Yellow-Brown  Blue        Yes                                         E       Red-Brown     Red-Brown   No                                          F       Red-Brown     Red-Brown   No                                          ______________________________________                                    

Mixtures of lower alcohols failed to inhibit the polymerization reactionin solution. The solvent mixture of 1-butanol and acetone also failed toinhibit the polymerization reaction. On the other hand, mixtures ofalcohols and basic solvents such as pyridine and tetrahydrofuraninhibited the polymerization reaction in solution.

Films of PEDT were cast from some of the solutions of Table 1. The filmswere prepared by placing some of the solution on a glass slide andheating to 110° C. for 10 minutes. Films prepared from solutions A, B,C, D, and F produced light blue conductive films of PEDT. Films preparedfrom the pyridine-containing solution were yellow-green andnon-conductive. Tetrahydrofuran was the only low-boiling solvent testedthat inhibited the reaction of 3,4 ethylenedioxythiophene in solutionyet allowed a conductive film of PEDT to be produced.

Example 2

Solutions of 3,4 ethylenedioxythiophene (monomer), Fe(III) tosylate(oxidizing agent and dopant), and solvent(s) were made according to thecompositions given in Table 3. The monomer was obtained from Bayer underthe trade name Baytron M, and the Fe(III) tosylate was obtained fromBayer as a 50 wt % solution in 1-butanol (trade name Baytron C). Herethe 1-butanol is the high-boiling solvent (BP 117° C.). The othersolvent was selected from the classes of alcohols, cyclic ethers, andfurans that have a boiling point lower than that of 1butanol.

Each of the solutions was placed in a closed vial. The solutions wereallowed to stand at room temperature for eight hours. After this timeperiod, the solutions were examined visually for color changes and alsofor the presence of precipitated PEDT powder. As the polymerizationreaction proceeded, the color of the solution became blue, and blueprecipitate of PEDT polymer formed.

                  TABLE 3                                                         ______________________________________                                        Solution Composition (by weight %)                                            Component       G        H     I     J   K                                    ______________________________________                                        3,4 ethylenedioxythiophene                                                                    6        6     6     6   6                                    Fe(III) tosylate                                                                              24       24    24    24  24                                   1-butanol       24       24    24    24  24                                   2-propanol      46       0     0     0   0                                    Tetrahydrofuran 0        46    0     0   0                                    2-methyl tetrahydrofuran                                                                      0        0     46    0   0                                    Tetrahydropyran 0        0     0     46  0                                    2,5 dimethylfuran                                                                             0        0     0     0   46                                   ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Reaction of Solutions at Room Temperature                                     Solution                                                                              Initial Color                                                                              Color at 8 hours                                                                           Precipitate                                 ______________________________________                                        G       Yellow-Brown Dark Blue    Yes                                         H       Red-Brown    Red-Brown    No                                          I       Red-Brown    Dark Red-Brown                                                                             No                                          J       Red-Orange   Darker Red-Orange                                                                          No                                          K       Red-Yellow   Dark Green   Yes                                         ______________________________________                                    

Films were prepared from the solutions as in Example 1. Solutions G andH produced good quality blue PEDT films. Solutions I and K produced darkblue powdery material. Solution J produced a PEDT film of somewhatdarker color than did Solutions G and H. Both tetrahydrofuran andtetrahydropyran inhibited the reaction between Fe(III) and 3,4ethylenedioxythiophene in solutions containing more than 5 wt % monomerwhile allowing conductive films of PEDT to be formed.

Example 3

Solutions of 3,4 ethylenedioxythiophene (monomer), Fe(III) tosylate(oxidizing agent and dopant), and solvent(s) were made according to thecompositions given in Table 5. The monomer was obtained from Bayer underthe trade name Baytron M, and the Fe(III) tosylate was obtained fromBayer as a 50 wt % solution in 1-butanol (trade name Baytron C).

                  TABLE 5                                                         ______________________________________                                        Solution Composition (by weight %)                                            Component        L          M     N                                           ______________________________________                                        3,4 ethylenedioxythiophene                                                                     4          4     4                                           Fe(III) tosylate 16         16    16                                          1-butanol        16         16    16                                          Water            1          1     1                                           2-Propanol       63         0     0                                           Tetrahydrofuran  0          63    0                                           Tetrahydropyran  0          0     63                                          ______________________________________                                    

PEDT films were cast from each solution on mylar film using an ACCU-LABdraw-down rod from Paul N. Company (Model AP-JR 80). Surface resistancesare shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        Surface Resistance of Cast PEDT Films                                         Solution   Surface Resistance (ohms/sq)                                       ______________________________________                                        L          95                                                                 M          79                                                                 N          12,800                                                             ______________________________________                                    

The use of tetrahydrofliran in the solution allowed production of filmsof comparable surface resistance to films produced from solutions with2-propanol, but in the case of tetrahydrofuiran the solutions werestabilized against reaction. Tetrahydropyran also stabilized thesolutions against reaction, but PEDT films produced had higher surfaceresistances. Thus, tetrahydropyran is not as desirable a solvent astetrahydrofuran.

Example 4

Tantalum powder of charge 26,000 CV/g was pressed into pellets andsintered to form a porous body of dimensions 0.81 mm thick, 2.92 mmwide, and 3.94 mm long. The pellets (anodes) were sintered and thenanodized (formed) in a phosphoric acid electrolyte to 28 volts. Dippingsolutions with the compositions given in Table 7 were prepared.Conductive polymer was deposited in the anodized tantalum pellets bypolymerization of the dipping solution at 110° C. followed by washing inwater. The anodes were reformed after every 4th dip in a 1% aqueoussolution of p-toluenesulfonic acid. After 12 dips, a conductive polymerdispersion (Baytron P from Bayer) was applied followed by a silverpaint.

The electrical parameters of the capacitors are shown in Table 8. Bothtetrahydrofuran and tetrahydropyran stabilized the dipping solutionsagainst polymerization in the dipping bath. Solutions containingtetrahydrofuran could be used to produce capacitors with equalelectrical characteristics to those produced from solutions containingthe non-inhibiting 2-propanol.

                  TABLE 7                                                         ______________________________________                                        Composition of Impregnation Solutions (by weight %)                           Component           O        P      Q                                         ______________________________________                                        3,4 ethylenedioxythiophene                                                                        4        4      4                                         Fe(III) tosylate    16       16     16                                        1-butanol           16       16     16                                        Water               1        1      1                                         3-glycidoxypropyltrimethoxy silane                                                                0.5      0.5    0.5                                       2-propanol          62.5     0      0                                         Tetrahydrofuran     0        62.5   0                                         Tetrahydropyran     0        0      62.5                                      ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        Capacitor Performance                                                         Dipping                                                                              Capacitance                 Leakage                                    Solution                                                                             (microfarad)                                                                            DF (%)    ESR (ohms)                                                                            (microamps)                                ______________________________________                                        O      42.9      2.6       0.13    1.53                                       P      43.0      2.5       0.13    0.71                                       Q      22.2      37.2      1.15    0.26                                       ______________________________________                                    

Wherein DF is dissipation factor and ESR is equivalent seriesresistance.

Example 5

A solution was prepared having 6 wt % 3,4 ethylenedioxythiophene(monomer), 24 wt % Fe(III) tosylate (oxidizing agent and dopant), 24 wt% 1-butanol, and 46 wt % 1,4 dioxane. The monomer was obtained fromBayer under the trade name Baytron M, and the Fe(III) tosylate wasobtained from Bayer as a 50 wt % solution in 1-butanol (trade nameBaytron C). Here the 1-butanol was the high-boiling solvent (BP 117°C.).

The solution was placed in a closed vial. The initial color of thesolution was red-brown. The solution was allowed to stand at roomtemperature for 8 hours. After this time period, the solution wasexamined visually for color changes and also for the presence ofprecipitated PEDT powder. The color of the solution remained red-brownand no precipitate of PEDT polymer was formed.

A film of PEDT was cast from the solution obtained. The film wasprepared by placing some of the solution on a glass slide and heating to110° C. for 10 minutes. The film produced a light blue conductive filmof PEDT. The 1,4 dioxane low-boiling solvent inhibited the reaction of3,4 ethylenedioxythiophene in solution yet allowed a conductive film ofPEDT to be produced.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the compositions and methodsof the present invention without departing from the spirit or scope ofthe invention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

What is claimed is:
 1. A process for depositing a conductive polymer ina porous pellet comprising:(a) providing a porous pellet made from amaterial selected from the group consisting of tantalum, aluminum,niobium, zirconium, and hafnium; (b) anodizing the pellet; (c) dippingthe pellet into a solution comprising a monomer, an Fe(III) oxidizingagent and a mixed solvent, the mixed solvent comprising a first solventhaving a first boiling point, wherein the first solvent does notappreciably complex with Fe(III), and a second solvent having a secondboiling point which is lower than the first boiling point, wherein thesecond solvent forms a weak complex with Fe(III); (d) evaporating thesecond solvent; (e) reacting the monomer with the oxidizing agent toform a conductive polymer; and (f) washing the pellet.
 2. The process ofclaim 1 wherein the first solvent is 1-butanol, and the second solventis tetrahydrofuran.
 3. The process of claim 1 wherein the first solventis selected from the group consisting of lower alcohols and ketones. 4.The process of claim 3 wherein the first solvent is 1-butanol.
 5. Theprocess of claim 1 wherein the second solvent is selected from the groupconsisting of cyclic ethers and dioxanes.
 6. The process of claim 5wherein the second solvent is tetrahydrofuran.
 7. The process of claim 1wherein the monomer is selected from the group consisting of pyrrole,thiophene and derivatives thereof.
 8. The process of claim 7 wherein themonomer is 3,4-ethylenedioxythiophene.
 9. The process of claim 1 whereinthe oxidizing agent is Fe(III) tosylate.
 10. The process of claim 1wherein the monomer is present in an amount of from about 0.5 to about12 percent by weight.